The Electromagnetic spectrum reference article from the English Wikipedia on 24-Jul-2004 (provided by Fixed Reference: snapshots of Wikipedia from wikipedia.org)

Electromagnetic spectrum

The term electromagnetic spectrum refers to the collection of possible wavelengths of electromagnetic radiation. Radiation at a particular wavelength λ (in vacuum) has an associated frequency ν and photon energy E. These quantities are related according to the equations:

and

where c is the speed of light (3×10⁸ m/s) and h = 6.65 × 10⁻³⁴ J·s is Planck's constant, or, in alternative units, h = 4.1 μeV/GHz.

The electromagnetic spectrum, shown in the table, extends from radio waves at the long-wavelength end to gamma radiation at the short-wavelength end, covering wavelengths from several miles down to fractions of the size of an atom.

In the branch of physics called electromagnetic spectroscopy, the spectra of radiation absorbed and emitted by matter is used to obtain information about matter.

Table of contents

1 Classifications 1.1 Radio Waves 1.2 Microwaves 1.3 Infrared radiation 1.4 Visible radiation (light) 1.5 Ultraviolet light 1.6 X-rays 1.7 Gamma rays 2 See also 3 External links

Classifications

While the classification scheme is generally accurate, in reality there is often some overlap between neighboring types of electromagnetic radiation. For example some low-energy gamma rays actually have a longer wavelength than some high-energy X-rays. This is possible because "gamma ray" is the name given to the photons generated from nuclear decay or other nuclear and subnuclear processes, whereas X-rays on the other hand are generated by electronic transitions involving highly energetic inner electrons. Therefore the distinction between gamma ray and X-ray is related to the radiation source rather than the radiation wavelength. Generally, nuclear transitions are much more energetic than electronic transitions, so usually, gamma-rays are more energetic than X-rays. However, there are a few low-energy nuclear transitions (eg. the 14.4 keV nuclear transition of Fe-57) that produce gamma rays that are less energetic than some of the higher energy X-rays.

Use of the radio frequency spectrum is regulated by governments. This is called frequency allocation.

Radio Waves

Radio waves cover the low-frequency, long-wavelength end of the spectrum. It is used for transmission of data, via modulation. Television, mobile phones, wireless networking and amateur radio all use it. Radio Waves can be detected at the Ultra High Frequency (UHF), Very High Frequency (VHF), Shortwave (HF or high frequency), Medium Wave (AM), Longwave, Very Low Frequency (VLF), and Extreme Low Frequency (ELF) bandwidth.

Microwaves

The extremely high frequency (EHF) of Microwaves come next. Microwaves are typically absorbed by molecules that have a dipole moment in liquids. In a microwave oven, this effect is used to heat food. Low-intensity microwave radiation is used in Wi-Fi.

It should be noted that an average Microwave oven in active condition is, in close range, powerful enough to cause interference with poorly shielded electromagnetic fields such as those found in mobile medical devices and cheap consumer electronics.

Infrared radiation

The infrared part of the electromagnetic spectrum covers the range from roughly 300 GHz (1 mm) to 400 THz (750 nm). It can be divided into three parts:

• Far-infrared, from 300 GHz (1 mm) to 30 THz (10 μm). The lower part of this range may also be called microwaves. This radiation is typically absorbed by so-called rotational modes in gas-phase molecules, by molecular motions in liquids, and by phonons in solids. The water in the Earth's atmosphere absorbs so strongly in this range that in renders the atmosphere effectively opaque. However, there are certain wavelength ranges ("windows") within the opaque range which allow partial transmission, and can be used for astronomy. The wavelength range from approximately 200 μm up to a few mm is often referred to as "sub-millimeter" in astronomy, reserving far infra-red for wavelengths below 200 μm.

• Mid-infrared, from 30 THz (10 μm) to 120 THz (2.5 μm). Hot objects (black-body radiators) can radiate strongly in this range. It is absorbed by molecular vibrations, that is, when the different atoms in a molecule vibrate around their equilibrium positions. This range is sometimes called the fingerprint region since the mid-infrared absorption spectrum of a compound is very specific for that compound.

• Near-infrared, from 120 THz (2.5 μm) to 400 THz (750 nm). Physical processes that are relevant for this range are similar to those for visible light.

Visible radiation (light)

Color	Wavelength interval	Frequency interval
red	~ 625-740 nm	~ 480-405 THz
orange	~ 590-625 nm	~ 510-480 THz
yellow	~ 565-590 nm	~ 530-510 THz
green	~ 520-565 nm	~ 580-530 THz
cyan	~ 500-520 nm	~ 600-580 THz
blue	~ 450-500 nm	~ 670-600 THz
violet	~ 380-430 nm	~ 790-700 THz
Continuous spectrum The spectrum of visible light

.

After infrared comes visible light. This is the range in which the sun and stars similar to it emit most of their radiation. It is probably not a coincidence that the human eye is sensitive to the wavelengths that the sun emits most strongly. Visible light (and near-infrared light) is typically absorbed and emitted by electrons in molecules and atoms that move from one energy level to another.

Ultraviolet light

Next comes ultraviolet. This is radiation whose wavelength is shorter than the violet end of the visible spectrum. It was discovered to be useful for astronomy by a Mariner probe at Mercury, which detected UV that "had no right to be there". The dying probe was turned over to the UV team full time. The UV source turned out to be a star, but UV astronomy was born. Being very energetic, UV can break chemical bonds. Chlorine will not normally react with an alkane, but give it UV and it reacts quickly. This is because the UV breaks the bond holding chlorine atoms into molecules of Cl₂. Lone atoms are extremely reactive and will react with the otherwise almost-inert alkanes. It also makes a mess of DNA, causing cell death at best and uncontrolled cell reproduction (cancer) at worst.

X-rays

After UV come X-rays. Hard X-rays are of shorter wavelengths than soft X-rays. X-rays are used for seeing through some things and not others, as well as for high-energy physics and astronomy. Black holes and neutron stars emit x-rays, which enable us to study them.

Gamma rays

After hard X-rays come gamma rays. These are the most energetic photons, having no lower limit to their wavelength. They are useful to astronomers in the study of high-energy objects or regions and find a use with physicists thanks to their penetrative ability and their production from radioisotopes. The wavelength of gamma rays can be measured with high accuracy by means of Compton scattering.

Note that there are no defined boundaries between the types of electromagnetic radiation. Some wavelengths have a mixture of the properties of two regions of the spectrum. For example, red light resembles infra-red radiation in that it can resonate some chemical bonds.

See also

• Spectroscopy : Electromagnetic spectroscopy

External links

• U.S. Frequency Allocation Chart - Covering the range 3kHz-300GHz (from Department of Commerce)
• Canadian Table of Frequency Allocations
• UK frequency allocation table (from Ofcom, which inherited the Radiocommunications Agency's duties, pdf format)

Electromagnetic Spectrum Radio waves | Microwave | Infrared | Visible light | Ultraviolet | X-ray | Gamma ray Visible: Red | Orange | Yellow | Green | Blue | Indigo | Violet

Radio spectrum ELF | SLF | ULF | VLF | LF/LW | MF/MW | HF/SW | VHF | UHF | SHF | EHF 3Hz | 30Hz | 300Hz | 3kHz | 30kHz | 300kHz | 3MHz | 30MHz | 300MHz | 3GHz | 30GHz | 300GHz

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